The purpose of this project is to search for novel breast cancer susceptibility genes. BRCA1 and BRCA2 genes do not account for all breast cancer families. Especially in certain regions of the world, their contribution to the breast cancer burden appears to be quite small. For example, in Finland only 25% of large breast cancer families are attributable to BRCA1 and BRCA2 mutations, raising the hypothesis that a possible third gene would be particularly prevalent in Finland. In this project, specimens from an existing collection of non-BRCA1/2 breast cancer families in Finland are used to carry out a genome-wide linkage search for additional breast cancer susceptibility genes. In addition, we will search for clues to perform targeted linkage analyses, such as to utilize comparative genomic hybridization to identify somatic losses of chromosomal regions in the cancer tissue. We will also attempt to stratify families by geographical region, by birth place of their grandparents, as well as by phenotypic characteristics of the tumors.